If you want to do heavy computation, you can easily call `compute()`. Dart VM manages a thread pool for that. A thread can execute multiple isolates though, its not a 1-1 mapping between threads and isolates.
IMHO, it was designed for developer productivity (not just speed of writing code), but JIT + AOT compilation to provide quick development but still fast apps for users, and as you can tell, many developers love it. An existing problem for Flutter is shader compilation jank, but this is being worked on by the Flutter team.
Read "Why did Flutter choose to use Dart?": https://flutter.dev/docs/resources/faq#why-did-flutter-choos...
I'm aware we have plenty of mediocre mainstream languages existing right now, but they all exist for historical reason and have rather useful ecosystems. However, to go out of the way to just make a new lame ass syntax, with zero ecosystem, and no distinguishable characteristic (which is why Flutter is always involved in selling it) why would you do that?
I'm all for new new paradigms, languages that are different like haskell or scala to flourish and teach us new ways of thinking, this is not it.
During the same time frame of Dart's existence, other languages that actually have innovative ideas were able to carve out their niches (go, rust, ts, kotlin, swift... list goes on), while Dart still has to rely on some UI toolkit to sell.